1. Field of the Invention
This invention relates in general to a method for holographic recording of information in the form of electrical signals and then retrieving the information, wherein a coherent object wave, which is pulse modulated, is recorded with a coherent reference wave.
2. Description of the Prior Art
The method of storing information in the form of electrical signals on a light sensitive medium or tape has been known. For instance, it has been proposed to sequentially record electrical signals in the form of one dimensional holograms in which a laser beam is utilized to provide a reference wave and an object wave with the object wave being pulse modulated with the signal to be recorded and the object and refence wave being superimposed on a moving recording medium to record a one dimensional hologram. For retrieval of the information, the storage medium is moved past a reference light source to reconstruct the points of the object wave which directed onto a detector to produce alternating current which after appropriate low-pass filtering corresponds to the original signal.
So as to achieve the highly theoretical maximum possible linear storage density of about 10.sup.3 bit/mm, the resolution of the reconstituted image points must be as high as possible. This means that the focal spot diameter of the reconstituted image points must be as small as possible. However, this means that the reconstituted image point must have a correspondingly small depth of field and in the limiting instance, this may mean a dimension in the order of magnitude of the wave length of the laser light being used. The storage tape must, therefore, when being reproduced, be guided perpendicularly to the direction of transfer with accuracy in the order of microns so as to provide that the image points are separately received. Particularly where high transfer speeds of the tape are used, this requirement for the accuracy of the guidance of the tape presents a problem.